Do I Need a Vapor Retarder?

Energy Nerd: Most buildings don’t need polyethylene anywhere, except directly under a concrete slab or on a crawlspace floor.

How vapor drive works: During winter, moisture driven outward by interior warmth can enter walls and ceilings by diffusing as vapor through building materials (although it’s far more likely to be carried by air leaks). This moisture can condense on the cold back of the exterior sheathing, causing problems such as mold and rot.

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Every couple of weeks, someone emails me a description of a proposed wall assembly and then asks, “Do I need a vapor retarder?” The short answer I give is that if the wall doesn’t have a vapor retarder, then there probably is no need to worry. It’s rare for a building to have a problem that’s caused by water-vapor diffusion. (Vapor retarders slow vapor movement and have a permeance rating of between 0.01 and 1. With a permeance of less than 0.01, vapor barriers essentially halt vapor flow.) Water vapor is water in a gaseous state — that is, water that has evaporated. The passing of water vapor through building materials is known as vapor diffusion.

Article Comments

If a vapor barrier membrane was applied to the exterior side of sheathing would there be concern for the ability of the sheathing to dry during cold or hot seasons particularly in climate 6? The wall assembly I'm thinking of goes like this from exterior to interior....cladding, rain screen, 2-3 lays of staggered joint XPS, polyurea membrane applied, SIP panel, drywall, paint. Thanks for the article, very informative!